Various types of active illumination cameras or imaging systems, generically referred to as “active illumination cameras”, that comprise their own light source for illuminating a scene that they image with “artificial light”, are known. Among such active illumination cameras are the ubiquitous flash cameras, gesture recognition cameras, and three-dimensional (3D) cameras. Gesture recognition cameras illuminate a person to image and recognize the person's gestures. 3D cameras image a scene with light they generate to determine distances to features in the scene. Depending on the mode of operation of a 3D camera, light provided by the camera's light source may be spatially modulated light referred to as structured light, such as typically used by triangulation type 3D cameras, or temporally modulated light, such as light transmitted in pulses, typically used by various types of time of flight (TOF) 3D camera.
For many applications, such as for tracking a person's gestures to interface the person with a computer, preferred design specifications for an active illumination camera can be antagonistic, and accommodating such competing design preferences can be expensive.
For example, for gesture tracking and/or 3D imaging to interface a person with a computer game, it is often desirable for an active illumination camera to have a relatively large field of view (FOV) characterized by a wide view angle, in which the person can move freely and still accurately be imaged by the camera. The FOV of a camera is a region of space defined by a solid angle that extends from an optical center of the camera and for which points therein are imaged by the camera's optical system on a photosensitive sensor, hereinafter a “photosensor”, that the camera comprises. A view angle of a camera's FOV is a largest possible angle between lines that lie in the camera's FOV and extend from the camera's optical center. A view angle may be defined for any plane that intersects the camera's optical center. View angles are generally defined for planes that contain the camera's optical axis. Practical view angles for imaging human activities are usually horizontal and vertical view angles defined for planes respectively parallel and perpendicular to the ground. It can be advantageous for the FOV to be characterized by a wide view angle, often a wide horizontal view angle as large as 90°, 120°, or 150°.
To provide the camera with a wide angle FOV and accurate imaging, the camera usually has an optical system comprising a lens or lens system having a small effective focal length “f”, and a relatively large photosensor, having a large number of photosensitive pixels. An effective focal length of an optical system is a focal length of a thin lens equivalent of the optical system that can be used to represent functioning of the optical system.
However, illuminating a large FOV with light from the camera's light source is generally both technically and cost-wise challenging. Intensity of illumination provided by the light source is usually limited by cost considerations and heat dissipation requirements for maintaining the light source, and camera, at an acceptable operating temperature. Amounts of light from the light source reflected by the person and other features in the camera's FOV are therefore usually limited.
To compensate for limited illumination, the camera may have enhanced light collecting efficiency and registration capacity so that amounts of reflected light registered by pixels in the camera's photosensor are sufficient for signals the pixels generate to have acceptable signal to noise ratios (SNRs). Light collecting efficiency is a measure of an intensity (optical energy per unit area) of light imaged on the camera photosensor from that portion of light collected by the camera lens per unit area of an object that the camera images. Light registration capacity is a measure of how much signal that a pixel in the camera's photosensor produces per unit of optical energy that the camera images on the pixel and has units of signal magnitude per unit of optical energy. A product of a camera's light collecting efficiency and light registration capacity is a measure of the camera's sensitivity to light from a scene that it images and is referred to as the camera's light acquisition sensitivity (LAS).
Light collecting efficiency and registration capacity can be enhanced by lowering the f number (f#) of the camera lens and increasing the size of pixels in the camera's photosensor. A lens f# is equal to the lens's focal length, f, divided by a diameter, D, of its aperture—that is f#=f/D. Aperture diameter D may be controlled by any of various diaphragms and stops. A minimum f# refers to an f# for a maximum possible D, usually a diameter close to a physical diameter of the lens.
Conventional digital cameras that image a scene in daylight and/or with light from a conventional flash have FOVs characterized by view angles between about 40° and about 60°, comprise square pixels having side dimensions between 1.2μ-6μ (microns), and minimal f#s equal to between 2.8-3.5. For gesture recognition and multiplayer video game applications on the other hand, it can be advantageous for an active illumination camera having a wide angle FOV to have an f# less than about 2, and large pixels having a side dimension greater than or equal to about 7.5 microns.
However, decreasing a camera's f# and increasing its pixel size generally decreases camera resolution and introduces optical distortions in images acquired by the camera unless the camera's optical system is specially designed to compensate for the distortions. Configuring the camera to moderate optical distortions can be technically difficult and involve costs that price the camera out of its intended market.